Understanding light power values
According to the documentation the strength value of the node should be in Watts, but this doesn't seem to be correct from experimentation.  The empirical behaviour of this seems to be as a multiplier.  So if you have a 1000W light and the strength set to 0.5 then you should have the effect of a 500W light.

Regarding color it is important that you set your light color to white if you intend to set the color of the node, this works in much the same way.

Concretely, it appears that nodes filter/amplify the light itself.  You can check this by setting you light power to 0W, no matter what you set the node values to, do you'll not produce any light.

I'm not sure what the intended behaviour is, but playing around with it suggests that a white emission shader of strength 1.0 will produce light exactly as it is configured in the light properties panel.

So the answer to your question is both can be used, it is up to you to decide if it is necessary to use nodes at all for this and just set the properties on the area light as you would expect.

Scale will matter as this defines the falloff of the light, however if you change the scale, blender will automatically compensate by changing the power level of the light.  So if you set everything up and then change the scale you shouldn't notice any difference unless you look at the power property of the light.